1. Field of the Invention
The present invention relates to a tracking controller of an apparatus for optically recording a signal into a recording medium or reproducing the signal recorded in the recording medium by using a light source of a laser or the like, particularly for performing control so that a light beam of the optical recording/reproducing apparatus accurately scans tracks on a recording medium.
2. Description of the Related Art
In recent years, it is known that tracking is performed not by moving the whole of an optical system but by separating a part of the optical system, setting it to a movable part, and moving the movable part, namely, the optical head in order to decrease the depth of a recording/reproducing apparatus and realize high-speed retrieval. Moreover, to decrease the weight of a movable part, an optical recording/reproducing apparatus is known in which a galvano-mirror recorder is used as a tracking actuator and the galvano-mirror recorder is set to a stationary part. A conventional tracking controller is described below by referring to FIGS. 1 and 2.
FIG. 1 is a block diagram showing the constituents of the conventional tracking controller. A light beam 108 emitted from a light source 101 of a semiconductor laser or the like is transformed into parallel rays by a collimator lens 102 and thereafter the parallel rays pass through a beam splitter 103 and are reflected by a galvano-mirror recorder 119 serving as a fine tracking actuator.
The rays reflected by the galvano-mirror recorder 119 are further reflected by a mirror 104 arranged at a movable part and thereafter they are converged by an object glass 105 and applied onto a rotating disk 107. The disk 107 connects with a spindle motor 106 for rotating the disk 107.
A light beam reflected by the disk 107 passes through the object glass 105 and then it is reflected by the mirror 104 and galvano-mirror recorder 119 and reaches the beam splitter 103. The light beam reaching the beam splitter 103 is reflected toward a convex lens 109. After the light beam passes through the convex lens 109, it is split into light beams 111 and 115 by a cylindrical polarization beam splitter 110 (hereafter referred to as cylindrical P.B.S.).
One split light beam 111 is converged on a dual-face photosensor 112. Outputs A and B corresponding to the luminous energy of the converged light beam emitted from the two-split face are inputted to each terminal of a differential amplifier 114. A track error signal is obtained by performing the operation for obtaining the difference between the outputs A and B of the photosensor 112 with the differential amplifier 114. The method for thus detecting the track error signal as an output of the differential amplifier 114 is disclosed in the official gazette of Japanese Patent Laid-Open No. Sho 49-60702 and is known as the push-pull method.
The outputs A and B coming from the two-split face of the photosensor 112 are also inputted to an adding amplifier 116. The sum of the outputs A and B and a luminous energy sum signal are obtained by the adding amplifier 116.
The track error signal outputted from the differential amplifier 114 is inputted to a variable amplifier 117. The gain of the variable amplifier 117 is adjusted so that the amplitude of the track error signal at the output point "a" is kept almost constant. The output of the variable amplifier 117 is inputted to a divider 118. The luminous energy sum signal is also inputted to the divider 118 from the adding amplifier 116. BY dividing the output of the variable amplifier 117 by that of the adding amplifier 116, the amplitude of the track error signal is kept almost constant against a change of the luminous energy of the light beam or a change of the reflectance of the disk 107 when recording or deleting data.
The other split light beam 115 split by the cylindrical P.B.S. 110 is converged on the quad-face photosensor 112. A focus error signal for detecting that the light beam on the disk 107 has deviated from a predetermined convergent state is obtained in accordance with the output of the four-split face. In this case, the focus error signal is detected by using the differential amplifier 113 and by means of the known astigmatism method. The light beam is controlled so that it is applied onto the disk 107 under the predetermined convergent state by the known focus control for driving a focus actuator (not illustrated) in accordance with the focus error signal. A detailed description of the above focus control is omitted because the focus control is not directly related to the present invention.
The operation of the entire optical system when the light beam converged by the object glass 105 is controlled so that it is accurately applied onto a target track, that is, when tracking control is performed is briefly described below. This tracking control is performed by mainly driving the galvano-mirror recorder 119 serving as a fine tracking actuator at a high frequency and mainly driving a linear motor 120 serving as a coarse tracking actuator at a low frequency. Retrieval for moving a light spot in a wide range covering the entire area of the disk 107 is also performed by driving the linear motor 120.
As described above, the track error signal whose amplitude is kept almost constant against a change of the luminous energy of the light beam or a change of the reflectance of the disk 107 by the divider 118 is inputted to a phase compensation circuit 122 of a tracking servo loop (hereafter referred to as TR servo loop). The output of the phase compensation circuit 122 of the TR servo loop is inputted to one input terminal of a signal selection circuit 127 for selecting whether to perform tracking control in which the light spot applied onto the disk 107 is controlled so as to follow tracks on the disk 107 or to execute the lock servo operation for securing the galvano-mirror recorder 119 to a desired position. A signal outputted from a phase compensation circuit 128 for compensating the phase at the gain intersection point of the lock servo loop is inputted to the other input terminal of the signal selection circuit. An output terminal of the signal selection circuit 127 is connected to a driving circuit 126 for driving the galvano-mirror recorder 119. When a signal sent from the phase compensation circuit 122 of the TR servo loop is selected by the signal selection circuit 127, the galvano-mirror recorder 119 is rotated by the output of the driving circuit 126 corresponding to the track error signal. The direction for the light beam to be reflected changes due to rotation of the galvano-mirror recorder 119 and the light spot moves in the direction crossing the tracks on the disk 107 (hereafter referred to as the tracking direction) so that it is located on the track. Thus, the light spot is controlled so that it is continuously located at the center of a target track.
The mirror 104 and object glass 105 are mounted on the linear motor 120 capable of moving in the track direction from the inside to outside perimeters of the disk 107. This mechanism constitutes an optical head. The light spot moves from the inside to the outside perimeters of the disk 107 in the tracking direction in accordance with movement of the linear motor 120. When tracking control is performed, the output of the phase compensation circuit 122 of the above TR servo loop is inputted, via the equivalent filter 123, to the phase compensation circuit 124 used to control the linear motor 120. The equivalent filter 123 has a characteristic approximately equal to the input/output characteristic of the galvano-mirror recorder 119 serving as a fine tracking actuator, that is, the rotational characteristic for input. The output of the phase compensation circuit 124 is inputted to a driving circuit 125 for driving the linear motor 120 and the linear motor 120 is controlled by the output so that the galvano-mirror recorder 119 can rotate while maintaining the natural state, that is, the state in which the optical axis center of the optical system is aligned with that of the light beam entering the object glass 105.
A lock servo for preventing the galvano-mirror recorder 119 set to the stationary part from vibrating due to a disturbance and securing it to a predetermined position when tracking control is not performed or when the linear motor 120 performs operations of retrieval in the tracking direction on the disk 107 is described below by referring to FIGS. 1 and 2. FIG. 2 is a block diagram of a conventional lock servo. The constituents of the lock servo are described below by referring to FIG. 2. A signal detected by a reflection-type photosensor 121 for detecting a rotation angle of the galvano-mirror recorder 119 is inputted to the driving circuit 126 of the galvano-mirror recorder 119 through the phase compensation circuit 128 and signal selection circuit 127 of the lock servo loop and returned to the galvano-mirror recorder 119 in the form of negative feedback. The lock servo is described below again by referring to FIG. 1. The rotation angle of the galvano-mirror recorder 119 is detected by the reflection-type photosensor 121 and returned as an input of the galvano-mirror recorder 119 in the form of negative feedback through the phase compensation circuit 128 for compensating the phase at the gain intersection point of the lock servo. By executing the operation of the lock servo comprising the above constituents, it is possible to secure the galvano-mirror recorder 119 to a desired position in accordance with a rotation angle detection signal sent from the reflection-type photosensor 121.
For the conventional tracking controller shown in FIG. 1, however, the galvano-mirror recorder 119 is used as a fine tracking actuator, and moreover an optical head is constituted by setting the galvano-mirror recorder 119 to the stationary part and arranging the mirror 104 and object glass 105 on the linear motor 120 serving as a coarse tracking actuator. Therefore, the length of the optical path from the galvano-mirror recorder 119 to the object glass 105 via the mirror 104 increases. When the optical path gets longer, the optical axis of the optical system is easily deviated from the center of the light beam if the attitude of the galvano-mirror recorder 119 under the non-controlled state changes from the attitude under the initial state i.e., the state in which the optical axis of the optical system is aligned with the center of the light beam due to rotation on the influence of gravity. The deviation of the optical axis from the center of the light beam is defined as optical axis deviation in this specification. When the optical axis error occurs, an offset occurs in the track error signal. The offset increases as the optical path gets longer.
The optical axis deviation caused by rotation of the galvano-mirror recorder 119 is described below by referring to FIG. 3. As shown in FIG. 3, when the galvano-mirror recorder 119 rotates, the above optical axis deviation occurs and the track error signal has an offset. In other words, when tracking control is performed, the optical axis deviation occurs due to rotation of the galvano-mirror recorder 119. Even when tracking control is not performed, the optical axis shifts in the rotational direction similarly to the case in which tracking control is performed if the attitude change of the galvano-mirror recorder 119 described above occurs. Therefore when the optical axis deviation occurs, a large offset occurs in the track error signal due to spherical aberration of the object glass 105, coma aberration of luminous flux, or eclipse due to a lens rim. If tracking control is performed under the above state, the offset already generated in the track error signal is further increased because the galvano-mirror recorder 119 is further rotated.
If the optical axis deviation increases when tracking control is not performed, the offset of the track error signal also increases. This type of offset may not be removed by a circuit behind the circuit for detecting the track error signal because the offset gets too large and thereby the circuit is saturated. Moreover, the amplitude of the track error signal may decrease or the track error signal may disappear because of eclipse or the like due to the lens rim. When the track error signal disappears, it is impossible to make the track error signal appear again even if the offset is removed by the circuit.
If the track error signal has an offset when tracking control is performed, the light spot on the disk 107 is controlled by a tracking control system so that it is located at a position deviated from the center of a target track. That is, the light spot on the disk 107 is controlled into an off-track state.
If the above off-track state occurs due to rotation of the galvano-mirror recorder 119 when tracking control is performed,a problem on degradation of the tracking accuracy occurs such that, for example, the recording characteristic for recording data in the disk 107 or the reproducing characteristic for reproducing the data is degraded or a light spot is easily deviated from a track.
Moreover, if the attitude of the galvano-mirror recorder 119 changes due to a variation with time or a change of environmental temperature when tracking control is not performed, the center of the light beam is deviated from the optical axis of the optical system and an offset occurs in the track error signal. If an optical axis deviation occurs due to a variation with time or a change of environmental temperature, a large offset may occur in the track error signal or the track error signal may disappear. When the track error signal has a large offset or it disappears, it is impossible to securely execute the operation of a tracking control system. Thus, problems occur that the recording/reproducing apparatus cannot be started and its reliability is greatly degraded.
When using the reflection-type photosensor 121 described in the Prior Art as a rotation angle detector of the galvano-mirror recorder 119, a signal outputted from the reflection-type photosensor 121 includes a direct-current offset (hereafter referred to as a DC offset). Moreover, the detection characteristic of the reflection-type photosensor 121 changes due to a temperature change or a variation with time and the DC offset changes. When the DC offset occurs in the output of the reflection-type photosensor 121 or it changes, the locked position of the galvano-mirror recorder 119 changes by a value equivalent to the DC offset. When the locked position of the galvano-mirror recorder 119 changes, the optical axis of the optical system is deviated. This optical axis deviation causes a large offset in the track error signal.
When the operation of the lock servo of the galvano-mirror recorder 119 is executed for a large DC offset of the reflection-type photosensor 121, the galvano-mirror recorder 119 is locked at a position where an optical axis deviation occurs and an offset occurs in the track error signal or the amplitude of the track error signal decreases. Thereby, counting errors easily occur under retrieval.